The primary purpose of this multi-center, 4-part, Phase I study is to examine the pharmacokinetics of medications used frequently to prevent opportunistic infections in AIDS patients. First, the pharmacokinetics of single agents (sulfamethoxazole or dapsone) will be examined. Second, we will study the kinetics of these agents when used in combination with a second agent fluconazole or clarithromycin or rifabutin) to determine whether these second drugs affect the AUC and clearance of N-hydroxy metabolites and parent drugs. The secondary objectives of this study are to compare the degree of inhibition of clearance of N-hydroxy metabolites in patients to in vitro predictions and to determine if any single treatment alters the total clearance or renal clearance of the parent drug. Based on preliminary studies in volunteers, we hypothesize that the AUC of the parent drugs (sulfamethoxazole or dapsone) will increase with the co-administration of fluconazole and clarithromycin and that the AUC of N-hydroxy metabolites will decrease. Changes should be in the opposite direction when rifabutin is administered.